METHOD, SYSTEM, AND APPARATUS FOR THE GROWTH OF ON-AXIS SiC AND SIMILAR SEMICONDUCTOR MATERIALS

ABSTRACT

A novel approach for the growth of high-quality on-axis epitaxial silicon carbide (SiC) films and boules, using the Chemical Vapor Deposition (CVD) technique, is described here. The method includes a method of substrate preparation, which allows for the growth of “on-axis” SiC films, plus an approach giving the opportunity to grow silicon carbide on singular (a small-angle miscut) substrates, using halogenated carbon-containing precursors (carbon tetrachloride, CCl 4 , or halogenated hydrocarbons, CHCl 3 , CH 2 Cl 2 , or CH 3 Cl, or similar compounds or chemicals), or introducing other chlorine-containing species, in the gas phase, in the growth chamber. At gas mixtures greater than the critical amount, small clusters of SiC are etched, before they can become stable nuclei. The presence of chlorine and the formation of gas species allow an increased removal rate of these nuclei, in contrast to the growth without the presence of chlorine. Or, alternatively, the novel precursors introduced in the growth system reduce the effective supersaturation ratio of the Si species in the growth layer. The reduction of the supersaturation ratio reduces or eliminates the 2D (and 3C—SiC) nucleation which would occur due to the large terrace widths present on the on-axis wafers. This allows the growth of Silicon Carbide epitaxial layers on SiC substrates or composite substrates with monocrystalline layers. This can also be applied to the other semiconductors, chemicals, compounds, materials, growth methods, or devices.

RELATED APPLICATIONS

The current application is related to the two co-pending applicationSer. Nos. 11/626,388 and 11/626,387, both filed Jan. 24, 2007, bothowned by one of the assignees of the current application (WidetronixInc., Ithaca, N.Y.), and both have the same inventors as the ones in thecurrent case: Yuri Makarov and Michael Spencer. The entire contents ofwhich are incorporated herein, by reference.

FIELD OF THE INVENTION

The disclosure relates to the semiconductor materials. Morespecifically, the disclosure relates to the epitaxial growth of siliconcarbide and similar materials, by chemical vapor deposition and othergrowth methods.

BACKGROUND OF THE INVENTION

Silicon Carbide

The superior properties of silicon carbide, as compared with silicon,make it a perspective material for high power and high-temperatureelectronics (e.g. high-power transistors, thyristors, devices withP-type and N-type conductivity layers (e.g. diodes), and rectifiers).Due to an extremely high thermal conductivity (3.9 W/cm*K for SiC, vs.1.3 for Si) and high breakdown voltage (1 MV/cm for SiC, vs. 0.3 MV/cmfor Si), the SiC-based device structures are able to operate at muchhigher terminal voltages and output powers. The wide bandgap of SiC(>3.0 eV for hexagonal SiC, vs. 1.1 eV for Si) provides a low leakagecurrent of the p-n junction, even at high temperatures. In addition, SiCexhibits a remarkable mechanical and chemical stability.

Despite the obvious advantages, wide-scale application of SiC in thedevice industry is currently hindered by difficulties arising inmanufacturing of SiC-based structures of the required high quality (andby their high costs). The improvement of the quality of the growingepitaxial layers seems to be the most important task at the moment. Thistask includes the achievement of a good surface morphology, highthickness uniformity, an accurate stoichiometry, and a low defectdensity of the epilayers.

One item that presently hinders the realization of stable bipolardevices (for example) is stacking faults, which are generated from basalplane dislocations that have propagated from the substrate into theactive region of the device during epitaxial growth. While it isdifficult to suppress the nucleation of stacking faults, if the epitaxyis performed on on-axis substrates the basal plane dislocationsterminating in the active region will be substantially reduced due togeometrical considerations. Considering on-axis substrates, the basalplane dislocations are more efficiently converted into relativelyharmless threading edge dislocations, as opposed to off-axis substrategrowth where many basal plane dislocations remain in the activestructure and are subsequently converted into stacking faults duringdevice operation. In on-axis growth, basal plane defects are effectivelyconverted, resulting in improved device performance and reliability.

Defects in SiC

Commercial quality SiC wafers and epilayers include threading screw,threading edge, and basal plane (which can have edge and screwcomponents) dislocations. Threading dislocations propagate with acomponent parallel to the c-axis, while dislocations that lieperpendicular to the c-plane are termed basal-plane dislocations. InSiC, it is energetically favorable for the basal plane dislocations todecompose into partial dislocations which bound a planar stacking faultdefect. These stacking faults, if present in the active region of thedevice, result in devices with functional properties that can changeunpredictably during operation. However, if the basal-plane dislocationsare efficiently converted into threading edge dislocations, the “killer”stacking faults will not be generated. Efficient conversion of basalplane dislocation to threading edge dislocation can occur if on-axisepitaxy can be accomplished.

Polytypes of SiC

Silicon carbide can form into over 150 different polytypes. The mainforms are 4H, 6H, and 3C (the cubic form). In the absence of growthsteps, the 3C polytype forms during epitaxy. Growth steps are producedby screw dislocations, substrate miscut/cut, or preferential etching.

On-Axis Epitaxial Growth of 4H—SiC and 6H—SiC

High temperature epitaxial growth of 4H—SiC and 6H—SiC is normallyperformed using wafers which have been miscut at an angle of 8 or 4degrees, respectively, toward the (1100) or (1,1,2b,0) direction. Themiscut substrates are used in order to produce a high density of growthsteps which are available for atomic attachment at kink sites located onthe steps. It was found that if “on-axis” wafers are used, the growth isthree dimensional due to high supersaturation of the growth layer. As aresult a high density of 3C polytype inclusions or a poor morphology dueto 2D nucleation will be produced.

SUMMARY OF THE INVENTION

In one aspect, the disclosure relates to a system, method, and apparatusthat improve the quality of the semiconductor materials used inelectronic devices, particularly power electronic devices relative towhat is presently used. In various embodiments, the disclosure relatesto an improved process for minimizing crystal defects in silicon carbideepitaxial material and the resulting improved structures and devices.The described technique can also be applied to the similar structures,devices, material systems, and growth methods.

Growth on the On-Axis Substrate

In one aspect, the disclosure provides for growth of SiC on on-axiswafers. This type of growth is also important for the production ofsubstantially drift-free PIN diodes and other high power devices,fabricated from SiC. It is generally accepted that polytype replicationduring SiC growth is accomplished by a process known as step-flowepitaxy. In the step flow growth mode, silicon atoms migrate along thesurface until they encounter a step-edge. At the step edge, the atomsare incorporated at kink sites into the crystal as described by the BCStheory [W. K. Burton, N. Cabrera and F. C. Frank, Philos Trans Roy. Soc.London A243, 299 (1951)]. The growth rate and surface morphology aredependant on the number of growth steps, supersaturation ratio of thegrowth species, and growth temperature. The interaction of theseparameters for SiC is discussed (using BCS theory) in detail by Kimoto[Tsunenobu Kimoto and Hiroyuki Matsunami Journal of Applied Phys. Vol 75Jan. 1994 pp. 850-859]. The substrate miscut determines the growth stepdensity and the growth terrace width. At the high miscut angle employedfor SiC growth (8 and 4 degrees), toward the 112b0 plane, the terracewidths are 17 angstroms and 36 angstroms, respectively.

Although we refer to “on-axis” wafers as singular (or zero degree off),in practice, they usually have a mis-cut of 0.1-0.2 degrees, or even asmuch/high as 1 degree off-axis (i.e. 0 to 1 degree range, off-axis). Thedistance between the steps here is significantly larger than that in thecase of SiC growth on “off-axis” wafers (i.e. more than 1 degreeoff-axis).

The range of angles for “small-angle off-axis” substrate/situation caneven go beyond/above 1 degree, mentioned above. However, for the patentprotection purposes, we present/use/specify/define/apply the followingrange of 0 to 10 degrees (small angles). Because for the small angles,the produced material exhibits substantially similar results (if notoptimum, as the case for exactly on-axis).

For example, for a wafer with a 0.2 degree miscut, the terrace width is721 angstroms, greater than 10 times the terrace width of the off-axissubstrates. At these large terrace widths, the supersaturation of thegrowth species is high, and the probability of 2D nucleation and 3Cpolytype formation is significantly increased. Additionally, macro-stepformation, due to step bunching, can occur. This phenomenon createsterraces with lateral dimensions of microns, further increasing theprobability of 2D nucleation. At high growth rates and with a deficiencyin the step edges, silicon atoms can form a cluster (before they willencounter a step edge). So, the terraces (regions between steps) can befilled with silicon clusters, which in turn cause 3C—SiC inclusions. Twoopposite processes occur here: further cluster growth due to theaddition of silicon atoms, migrating on the surface; and clusterevaporation, decreasing the cluster size. It is well-known that somecritical size of such clusters exists, and if these clusters grow beyondthis critical size, they will introduce regions of cubic SiC in thegrown layer. Thus, the introduction of cubic SiC growth is quite likelyduring the growth on on-axis wafers.

Growth on on-axis substrates also suffers from poor morphology anddefects due to the high supersaturation conditions of the growth. It iswell known from crystal growth theory that conditions of highsupersaturation produce 2D nucleation.

The addition of chlorine, such as from CCl₄, improves the materialquality, by eliminating Si clusters and reducing the supersaturationratio during epitaxial growth, as described below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1. The etching mechanism, preventing cubic SiC formation, duringSiC growth on “on-axis” wafer. CCl₄ is considered as chlorine-containingspecies, here.

FIG. 2. Simulated growth rate of SiC as a function of the flow ofHelium/HCl mixture. HCl is substantially 3% of the gas mixture.

FIG. 3 shows the surface of an on-axis growth produced under optimalconditions.

FIG. 4 shows the surface of a SiC wafer grown under sub-optimalconditions. (The features in FIG. 4 can be ascribed to 2D nucleationunder high supersaturation conditions. Under optimal conditions, asmooth featureless surface is obtained as shown in FIG. 3).

FIG. 5. Optical measurement of a sample. This data indicates that thesample is free from 3C—SiC polytype inclusions.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Chlorine addition provides two mechanisms for promotion of “on-axis”growth. First, chlorine provides an additional removal mechanism forsilicon clusters. FIG. 1 illustrates the scheme of such a mechanism. Theetching rate for silicon clusters is much higher than that for siliconcarbide fragments, formed at the step edges. Effective etching ofsilicon clusters by chlorine-containing species forms volatilecomponents and decreases average cluster size. So, one can expect thatsilicon clusters will be maintained below the critical size, for thefollowing silicon-to-carbon and silicon-to-chlorine ratios:

x ^((int))(Si)/x ^((int))(C)=0.7−1.3,

x ^((int))(Si)/x ^((int))(Cl)=0.03−1,

where x^((int))(Si), x^((int))(C), x^((int))(Cl) are the numbers ofsilicon, carbon, and chlorine atoms in the input gas mixture,respectively. As a result, cubic SiC growth will be suppressed onon-axis wafers, while polytype replication will be the dominant growthprocess. Thus, high quality SiC growth on the on-axis wafers becomespossible. As previously discussed, during on-axis growth, large sizeterraces are formed (due to step-bunching). If chlorine is used in thegrowth chamber, there is a simultaneous etching (which occurs during thegrowth). It has been experimentally shown that HCl etches the surface of“on-axis” SiC in such a way as to form periodic 6 bilayer steps. Thesesteps are closely spaced (˜0.3 micron), as compared to the terracesformed by step bunching (˜1-2 micron), and they could provide sites forstep flow growth.

A second mechanism for chlorine promotion of on-axis growth is thereduction of the supersaturation ratio during growth. Thesupersaturation ratio is the ratio of the number of adatoms in thegrowth layer to the number of adatoms in the growth layer underequilibrium conditions. In general, high quality crystal growth isperformed as close to equilibrium as possible. In SiC growth, the growthrate (GR) is limited by mass transport of the species through thestagnant layer. The relationship of the growth rate to step dynamics isgiven by equation 1 [see Tsunenobu Kimoto and Hiroyuki Matsunami,Journal of Applied Phys. Vol 75 Jan. 1994 pp. 850-859]. If chlorine isnot used, the only way to remove Si species from the growth surface isby evaporation (which is a function of the growth temperature). Ifexcess Cl is available, Si can be removed as a volatile chloride:

$\begin{matrix}{{GR} = {\left( \frac{2h\; \lambda_{s}}{\lambda_{0}n_{0}} \right)\left( {R - F_{Desorp}} \right)\tanh \; \left( \frac{\lambda_{0}}{2\lambda_{s}} \right)}} & (1)\end{matrix}$

The relationship between the supersaturation ratio and growth conditionscan be found from BCS theory and is shown in equation 2.

$\begin{matrix}{\alpha_{\max} = {{1 + {\frac{\lambda_{0}n_{0}R\; \tau_{s}}{2\lambda_{s}{hn}_{s\; 0}}\mspace{11mu} \tanh \; \left( \frac{\lambda_{0}}{\; {4\lambda_{s}}} \right)}} \approx {1 + {{A\left( \frac{1}{\lambda_{s}} \right)}^{2}\frac{R}{F_{Desorp}}}}}} & (2)\end{matrix}$

In these equations 1 and 2, R represents the arriving flux of Si atoms,λ_(s) represents the diffusion length, λ₀ represents the terrace width,no is the total density of adatoms sites on the surface, h is the stepheight, n_(so) is the adatom concentration at equilibrium, τ_(s) is themean residence time of Si atoms on the surface and A is a constant.F_(Desorp) represents the desorption flux. As detailed in our previouspatents, the growth rate of SiC is a function of chlorine in thereactant gases. FIG. 2 shows the simulated experimental growth rate as afunction of HCl flow. Note that for values of HCl where the ratio ofHCl/Si is greater than 9, the growth rate decreases. This indicates thatdesorption of Si species (F_(Desorp)) from the growth surface isincreasing. The important conclusion is that, for a given growthtemperature and growth rate (assuming λ_(s)˜constant), thesupersaturation ratio can be controlled by chlorine variation. Thisenables us to optimize the growth conditions independent of temperatureand growth rate, allowing us to obtain high quality “on-axis”homo-epitaxy. Without Cl addition, it is only possible to change thedesorption rate by increasing or decreasing the temperature (changingthe evaporation rate from the Si surface), as indicated earlier.

Experimental measurements of the diffusion length of Si on the surfaceof SiC have indicated that the diffusion length decreases as the SiCgrowth rate increases. We believe that this data indicates that thediffusion length is inversely related to the supersaturation ratio. Thiswas also observed by Nishizawa in his Si growth studies [J. Nishizawa,Y. Kato and M. Shimbo, J. Crystal Growth 31 290 (1975)]. Therefore, thesupersaturation ratio as a function of chlorine should change fasterthan the ratio of species arrival to species desorption.

FIG. 3 shows the surface of an on-axis growth produced under optimalconditions. FIG. 4 shows the surface of a SiC wafer grown undersub-optimal conditions. The features in FIG. 4 can be ascribed to 2Dnucleation under high supersaturation conditions. Under optimalconditions, a smooth featureless surface is obtained as shown in FIG. 3.

In the implementation shown and described herein, substrates which havebeen CMP-polished, followed by a finishing etch, to remove thesubsurface damage were used. The finishing is done by high temperatureHCl. FIG. 5 is the Raman measurement of an on-axis epitaxial film. Thelaser was moved to different positions, as indicated in the insert. Theresult shows that the scan from the epilayer has the same features asthe substrate, indicating no 3C inclusions.

The disclosure is not limited to specific compounds and elementsdiscussed above. Modifications in the types of halogenated carbonprecursors (such as halogenated hydrocarbons, e.g. CHCl₃ or CH₃Cl, ormethyl-containing compounds, such as CHCl₂—CH₃, and others) may beapplied, without departure from the spirit and scope of the invention.

The precursors can be any other chemical compound, element, or mixture,as long as the ratio, amount, or percentage of the decomposed speciesstay substantially the same or similar. In addition, the temperatures,flow rates, dimensions, and other design and growth parameters can bevaried, as long as the main objectives of the invention, mentionedabove, are more or less satisfied. It can also be applied tosemiconductors other than SiC and its related compounds.

In some embodiments, for the 0 to 1 degree range a mis-cut toward<1,0,-1,0> direction for the substrate orientation/cut can be used.

Note that an optimum result is obtained for a planar cut approximately0-1 degrees toward the principal axis, for the substrate, for theoptimum growth results.

Note that an optimum result is obtained for the off-axis, 0 to 1 degreeoff, along <1100> direction or <1,1,-2,0> direction, from the basalplane.

Note that more than one optimum sets of situations/parameters may existthat locally/relatively optimizes the growth quality for the epilayers,e.g. reduces the defect densities.

Note that for the extremely high Cl/Si ratios, as Cl increases, thegrowth rate goes down again, surface becomes rougher, and the materialquality degrades.

Note that different materials give the same type of results, with smallmodification of the parameters for the growth/substrate.

Note that Chlorine lets us go to a higher growth rate and to alower/smaller angle cut for substrate, with the same material quality.

Note that any Halogens, such as Fluorine and others, can also be usedfor these chemical compounds/reactions, instead of Chlorine.

Note that the growth rate curve versus the Cl/Si concentration ratiousually has a peak for the highest growth rate, for a given silane or Siconcentration. The height of the peak and the position of the peak aredependent on the silane or Si concentration. Thus, for the growth rate,we will obtain a family of curves which are changing based on silane orSi concentration. Thus, the range of the Cl/Si ratios mentioned above(or its reverse value, Si/Cl ratio) in this disclosure is only arelative ratio and should be considered within the context of the Siconcentration (for a specific or given Si concentration). Or,equivalently, it can be expressed based on (or relative to) the Carbonconcentration/range. Or, equivalently, it can be expressed based on (orrelative to) the Si/C concentration ratio (value, values, or range ofvalues). That is, it is expressed for a given value of (or correspondsto) Si/C ratio. This can be a single value or a range of values. Theoptimum condition(s) can be a single set or multiple sets ofparameters/conditions/ranges.

Note that for the above discussion, we have provided specificrange/values of Si/C ratio as an optimum range of parameters, as shownabove. However, it should be noted that this ratio or range is notunique, and it is dependent on the absolute value of the Siconcentration (i.e. the ratio is given or expressed, for a given Siconcentration).

The growth may be on multiple substrates. It can be on any shapesubstrate, e.g. flat or non-flat. It can be on any form substrate, e.g.in powder-form, gel-form, solid-form, or liquid-form.

Thus, in one of the embodiments, growth occurring without “enough” HCL(2D nucleation and 3C inclusions) can be ascribed to high localsupersaturation of the crystal. This high supersaturation is driven bythe high growth rate, as well as the large terrace widths of the on-axissubstrates. It is shown that the reduction in local supersaturation isdue to the increased “excess” Cl, which increases the desorption flux ofSi species in the form of volatile chlorides (indicated in thesimulations as a reduction in growth rate). In the absence of HCl,desorption of Si can be accomplished using thermal evaporation. Theupshot of this is that high growth temperatures are required in orderfor the crystal to growth close to equilibrium at high growth rates/andor on on-axis substrates. With HCl this requirement is removed.

Any variations of the teachings above are also meant to be covered andprotected by the current disclosure.

1. A system for the growth of the semiconductor material, said systemcomprising: A substrate holder, A chamber, and A gas source, Whereinsaid gas source is connected to said chamber, Wherein said gas sourcesupplies a gas to said chamber, Wherein said gas comprises a halogenelement or compound, Wherein said substrate holder holds a substrate,Wherein said substrate is an on-axis substrate, and Wherein saidsubstrate is a semiconductor substrate.
 2. A system as recited in claim1, wherein said substrate is a SiC substrate.
 3. A system as recited inclaim 1, wherein said halogen element or compound comprises chlorine. 4.A system as recited in claim 1, wherein said system is a chemical vapordeposition system.
 5. A system as recited in claim 1, wherein said gasis a halogenated carbon precursor.
 6. A system as recited in claim 1,wherein said system comprises multiple substrates.
 7. A system asrecited in claim 1, wherein said substrate is one or more of thefollowing forms: non-flat substrate, in powder-form, in gel-form, insolid-form, flat substrate, or in liquid-form.
 8. A system as recited inclaim 1, wherein said system is a system for the growth of deviceepilayers.
 9. A system as recited in claim 8, wherein said deviceepilayers are one or more of the following device structures: bipolardevice, PN junction, diode, PIN, transistor, rectifier, FET, device witha quantum well, device with a heterojunction interface, device withmultiple P and N layers, or laser.
 10. A system as recited in claim 8,wherein said device epilayers comprise one or more SiC layers.
 11. Assystem as recited in claim 8, wherein said device epilayers comprise oneor more wide bandgap semiconductor layers.
 12. A system as recited inclaim 1, wherein said semiconductor material comprises one or morepolycrystalline layers.
 13. A system as recited in claim 1, wherein saidsubstrate is cut or sliced in the range of 0 to 1 degree off the axis.14. A system as recited in claim 1, wherein said substrate is cut orsliced in the range of 0 to 10 degrees off the axis.
 15. A system asrecited in claim 9, wherein said one or more of the device structuresare for high temperature or high breakdown voltage operation orapplication.
 16. A system as recited in claim 1, wherein said substrateis prepared or modified by a chemical, compound, or agent which producesclosely-spaced step-producing growth surface for said substrate.
 17. Asystem as recited in claim 1, wherein said substrate is prepared ormodified by a chemical, compound, or agent which etches the growthsurface for said substrate.
 18. A system as recited in claim 1, whereinsaid substrate is prepared or modified by a chemical, compound, or agentbefore the growth process.
 19. A system as recited in claim 1, whereinthe ratio of concentrations of Si to C is optimized for better materialquality or growth process.
 20. A system as recited in claim 1, whereinthe ratio of concentrations of Si to Cl is optimized for better materialquality or growth process.